Electrographic recording system

ABSTRACT

An electrographic contact charging apparatus is provided comprising a roll containing a plurality of spaced electrodes in the surface thereof, said electrodes being disposed concentrically with the axis of rotation of said roll; a grounded electrode being spaced from said roll and parallel with the axis of rotation thereof and extending substantially coterminously with said roll; said grounded electrode and the electrodes in said roll being adapted to make contact in the nip formed therebetween with a dielectric medium adapted to pass therethrough; means for generating sequential informational triggering pulses; and commutating means for connecting the electrodes in said roll with said means for generating said triggering pulses. In another embodiment, a process is provided for continuous electrographic recording comprising passing an insulating web through the nip formed by a first roll containing a plurality of spaced electrodes in the surface thereof, said electrodes being disposed concentrically with the axis of rotation of said roll and at least one grounded electrode spaced from said roll and parallel with the axis of rotation thereof and extending substantially coterminously with said roll; forming electrostatic latent images on the web by selective charge transfer between the electrodes in said roll and said grounded electrode generated by sequential informational triggering pulses applied to the electrodes in said roll; and developing said electrostatic latent images.

Uited States atent Rittler et al.

[11 3,765,026 Oct. 9, 1973 ELECTROGRAPHIC RECORDING SYSTEM [73] Assignee: Xerox Corporation, Rochester,

[22] Filed: Nov. 3, 1970 [21] Appl. No.: 86,442

[52] US. Cl. r. 346/74 ES, lOl/DlG. 13

[51] Int. Cl G03g 15/00 [58] Field of Search 346/74 ES; l0l/D1G. 13

[56] References Cited UNITED STATES PATENTS 3,417,404 12/1968 Macouski 346/74 ES 3,196,012 7/1965 Clark 346/74 ES 3,414,723 12/1968 Pleitt 346/74 ES 3,060,432 10/1962 SChwertzm. 346/74 ES 3.441.938 4/1969 Markgraf .1 346/74 ES Primary Examiner-Howard W. Britton Attorney-James J. Ralabate, Laurence A. Wright, John E. Beck and William Kaufman [57] ABSTRACT An electrographic contact charging apparatus is provided comprising a roll containing a plurality of spaced electrodes in the surface thereof, said electrodes being disposed concentrically with the axis of rotation of said roll; a grounded electrode being spaced from said roll and parallel with the axis of rotation thereof and extending substantially coterminously with said roll; said grounded electrode and the electrodes in said roll being adapted to make contact in the nip formed therebetween with a dielectric medium adapted to pass therethrough; means for generating sequential informational triggering pulses; and commutating means for connecting the electrodes in said roll with said means for generating said triggering pulses.

In another embodiment, a process is provided for continuous electrographic recording comprising passing an insulating web through the nip formed by a first roll containing a plurality of spaced electrodes in the surface thereof, said electrodes being disposed concentrically with the axis of rotation of said roll and at least one grounded electrode spaced from said roll and parallel with the axis of rotation thereof and extending substantially coterminously with said roll; forming electrostatic latent images on the web by selective charge transfer between the electrodes in said roll and said grounded electrode generated by sequential informational triggering pulses applied to the electrodes in said roll; and developing said electrostatic latent images.

5 Claims, 2 Drawing Figures PATENTEU 9 5 aw Hm W Z /w W m Mm Hm m 1 ELECTROGRAPHIC RECORDING SYSTEM This invention relates to an electrographic recording system. More particularly, this invention relates to a system which enables charge transfer to be made between two or more electrodes to continuously form electrostatic charge patterns directly on an insulating medium for producing a viewable record.

The formation and development of images on the surface of photoconductive materials by electrostatic means is well known. The basic electrostatographic process as taught by C. F. Carlson, US. Pat. No. 2,297,691 involves depositing a uniform electrostatic charge on a photoconductive insulating layer, exposing the layer to a light and shadow image to dissipate the charge on the areas of the layer exposed to the light and developing the resulting electrostatic latent image by depositing on the image a finely divided electroscopic material referred to in the art as toner. The toner will normally be attracted to those layers of the photoconductive layer which retain a charge, thereby forming a toner image corresponding to the electrostatic latent image. This powder image may then be transferred to a receiving surface such as paper. The transferred image may subsequently be permanently affixed to the receiving surface by heat. Other suitable fixing means such as solvent or over-coating treatment may be substituted for the foregoing heat fixing step.

In electrographic recording, however, physical symbols or characters rather than light patterns are re corded as electrostatic images. The images are then developed by depositing thereon a finely divided powder, liquid developer, ink mist or the like. The resulting visible images can be made permanent by fusing or drying. Electrostatic images are produced by character faces, symbol-shaped electrodes or a configuration of dots each of which is formed by an intersecting electrode pair which are brought in close proximity to an insulating surface, such as a web of dielectric material. The web can be electrostatically precharged by an intense electric field to a point somewhat below a critical stress value. Transfer of the configuration of the symbol, character or dot from the electrode to the insulating web is effected by the use of a relatively low potential triggering pulse which raises the electric field above the critical stress value to produce a field discharge in the space between the insulating web and the electrode. The discharge action gives rise to the formation of an electrostatic, pattern of the symbol, character or dot on the insulating surface. Electronic switching circuits can be associated with the electrostatic apparatusto supply triggering pulses thereto in accordance with information received electrically from a digital computer or other signal source.

Thecritical stress defines that value of electric field strength at which breakdown occurs. It has been found that when there is such breakdown, a transfer or charge migration through the gap takes place. If, on the other hand, an electric stress is below the critical point, dielectric breakdown is not effected and there is no charge transfer. The point at which the charge will transfer can be determined empirically or by the use ofcritical stress curves. In the present invention, the web can be initially charged to a point below the critical value, the latent image being impressed on the precharged web by applying a triggering voltage to an electrode such as to raise the charge above the critical value.

As presently understood, the nature of the field discharge in the air gap is such that when critical stress is attained, ions which normally are present in the gap are accelerated into collisions with nearby air molecules thereby generating additional ions which similarly collide with further air molecules to create more ions, this action being cumulative. Charges are also released from the surfaces defining the gap by collisions with these surfaces by the moving ions. The travelling ions so produced deposit on the surfaces controlled by the electric field.

Since the ions created by the electric field are both positive and negative in polarity, the positively charged ions are propelled to the negative electrode surface, whereas the negatively charged ions move to the positive electrode surface. This results in neutralization of the charges which exist on the respective electrodes and also in the deposition of the new charges which raise the amount of charge deposited on the electrode surface. For example, where there exists negative charges on the surface and positive charges are moved thereto, neutralization takes place and the charge density of the negative surface is reduced. But if the surface is initially neutral and charges are moved thereto by the electric field in the gap, the deposition of additional or new charges takes place on the surface without neutralization, thereby raising the charge density as controlled by the field on the previously netural surface.

Thus air ionization which takes place in the gap creates a conductive region and allows charge flow between the surfaces defining the gap to effect a charge migration therebetween. This type of air ionization and air travel continues while the electric stress is above critical stress, but for a value below critical stress it has been found that once current flow through ion move ment in the gap has started, and as deposition takes place, the electric stress in the gap is reduced by the deposited charges which changes the field strength across the gap until charge migration stops.

' Several electrographic printing output devices have heretofore been developed for data-processing systems. These devices generally comprise a carriage which is advanced in a step-by-step manner in the printing direction. The carriage carries the printing elements which are usually segmented row or finger contact electrodes. This carriage carries the printing elements across the surface of the dielectric medium upon which the electrostatic images are being deposited. Upon arriving at the end ofa line, the printing elements are returned as quickly as possible to the beginning of the line. The time required for the return necessitates an intermittent operation and mitigates against high speed operation. Still further, it may be necessary to interrupt the operation of the transmitter during the time of the return of the printing elements in order that no textual material be lost. Additionally, and perhaps most importantly from a commercial viewpoint, the electrographic recording systems which have heretofore been employed, regardless of whether a movable or stationary array of finger contact electrodes are employed, are engaged in relative motion with the dielectric material resulting in wear and deterioration of the segmented row or finger contact electrodes necessitating frequent replacement.

Accordingly, it is an object of the present invention to provide a truly continuous electrographic recording system. It is another object of the present invention to provide. an electrographic recording system which obviates the need for segmented row or finger contact electrodes and the frequent replacement thereof.

These as well as other objects are accomplished in accordance with the present invention which provides an electrographic contact charging apparatus comprising a roll containing a plurality of spaced electrodes in the surface thereof, said electrodes being disposed concentrically with the axis of rotation of said roll; a grounded electrode spaced from said roll and parallel with the axis of rotation thereof and extending substantially coterminously with said roll; said grounded electrode and the electrodes in said roll being adapted to make contact with a dielectric medium adapted to pass therethrough; means for generating sequential informational triggering pulses; and commutating means for connecting the electrodes in said roll with said means for generating said triggering pulses. v

For a betterunderstanding of the invention as well as other objects and further features thereof, reference is had to the following detailed description to be read in connection with the accompanying drawing wherein:

FIG. 1 is a schematic diagram illustrating in perspective an electrographic recording system in accordance with the present invention.

FIG. 2 is a partial schematic diagram of an alternate embodiment of the present invention.

Referring now to the drawings and more particularly to FIG. 1, there is illustrated one embodiment of the continuous electrographic recording system of the present invention. More particularly, there is shown a web 10 of insulating medium traversing the electrographic recording system from feed roll 12 to windup roll 13. The web of insulating medium can comprise a polymer-coated paper or any other dielectric sheet having a sufficiently high resistance to hold an electrostatic image for a period of such duration as to permit subsequent utilization of the image by transfer to another surface or by development. Among the materials suitable for this purpose are polyethylene, cellulose acetate, ethyl cellulose, polyethylene terephthalate and dried or coated papers or the like having a resistivity in the order of about 10 ohm-centimeters. The web preferably has a thickness in the order of l to 2 mils.

As the insulatingweb 10 leaves supply roll 12, it can first pass through, an optional pre-stressing ionization zone designated generally as 14 which comprises an ionization electrode 16 housed in a shielding chamber 18 and connected to the adjustable slider of a potentiometer 20 whose center tap is grounded; a high voltage D.C. source such as batteryZZ being connected across the potentiometer element. A grounded conductive plate 24 bears against the back of the web l and is in electrical contact therewith within this zone whereby by varying the potentiometer slider relative tothe center tap, an electric field of a desired polarity and intensity may be established between the ionization electrode 16 and the web 10. The intensity of this field is adjusted so that the web is charged uniformly to a point somewhat below the critical stress value at which charge migration takes place.

The pre-charged or pre-stressed web 10 leaving the pre-charging stage 14 enters the electrographic recording station shown genrally as 26 wherein triggering voltages derived from means for generating sequential informational triggering pulses such as digital computer 30 are applied to the electrodes 35 in roll 31 to effect field discharge between said electrodes 35 and a grounded electrode in roll 29 causing selective charge transfer thereby forming an electrostatic latent image on the dielectric web 10 upon passage through the nip 27 formed by said rolls. if the potential of the triggering voltages is above the critical field strength, the prestressing of the web is considered unnecessary. The operation and structure of the electrographic recording station will be described in greater detail hereinafter.

As the web proceeds from the recording station 26, it passes through the development station generally designated 30 wherein the latent electrostatic image is rendered visible by depositing thereon a finely divided material, such as an ink mist, an electroscopic pigmented resin powder or other suitable developer which adheres to the charged image areas. A suitable powder development mechanism is fully disclosed for example in the Carlson, U.S. Pat. No. 2,357,809, and the Walkup and Fauser, U.S. Pat. No. 2,573,881, and a mechanism for developing electrostatic images with an ink mist is disclosed .in Carlson, U.S. Pat. No. 2,551,582.

After leaving the development station 30, the web bearing the developed image enters fusing station 32, the resin powder images are heated in a suitable oven to a temperature sufficient to fuse the resin. The fused image or the web is passed over tension roll 28 to windup roll 13.

If liquid ink development is employed, the fuser may serve merely as a heated dryer for the ink. With webs formed of polymeric materials which tend to distort when heated, it is preferred that the fuser consist of a chamber filled with air saturated with vapor or solvent for the resin images and not for the web material. Solvent is absorbed by the powder until it becomes tacky or semi-fluid and as the web leaves the fuser, the solvent evaporates leaving a fixed resin image on the web. With coated paper, a solvent for the polymer can be used, in which case it is possible to use infusible electroscopic powders, before they become imbedded in the polymer or wax coating which is softened by heat or solvent as it passes through the fuser.

Instead of fixing the powder images on the original web, it is possible to transfer the image to another sheet or surface by electrostatic transfer methods such as shown in Schaffert, U.S. Pat. No. 2,576,047, or by rolling against an adhesive coated surface. Thus, the developed images can be transferred and affixed to paper offset mats for use in offset printing or multiple copies. It is to beunderstood that any conventional means for development and transfer of electrostatic images can be employed for purposes of the present invention.

As shown in FlG. l, the pie-stressed insulating me- I dium l0 passes through the nip 27 formed between the pair of counter-rotating rolls 29 and 31 which together form the rotary electrographic contact charging apparatus of the present invention. Each roll contains a plurality of equi-spaced conductors designated respectively in each roll as 33 and 35 disposed in parallel relation and embedded or printed by electrochemical techniques or the like on the surface of the insulating rolls 29 and 31 which may be formed or coated with glass,'

a rigid plastic material or other suitable insulating material. The electrodes 33 and 35 in the respective rolls are orthogonally arranged with respect to each other and are adapted to have an intersecting locus comprising a plurality of crossover points in the nip 27. The same effect can be obtained by employing a stationary knife edge electrode in lieu of roll 29, said electrode extending across roll 31 parallel with the axis of rotation thereof.

in accordance with the present invention, any of the electrodes 35 in roll 31 can be activated pursuant to a sequential pulse transmitted from the digital computer 40 through commutating means 42. The electrodes 33 in roll 29 are adapted to be grounded in the nip 27 where they cross over electrodes 35 forming a plurality of cross-over points. The potential of the triggering pulse is highest at the cross-over points and falls off rapidly due to l/r dependence. Thus, an electrostatic point image can be transferred to the insulating medium at the point or points of virtual intersection of the activated electrodes. In other words, an image may consist of an array or plurality of uniform point images arranged in a desired pattern provided by the sequence of triggering pulses. For example, if a potential of the proper magnitude is applied to electrode (b) of electrode set 35 and if electrode n of electrode set 33 is grounded in the nip 27, an electrostatic point image will be transferred by charge migration to the insulating medium at the point of virtual intersection (X).

It has been found that the combined magnitude of the charge impressed upon the insulating medium in the pre-stressing zone and the charges impressed upon the insulating medium by the sequential pulse at the point of intersection of the activated electrodes and ground electrode in the nip is sufficient to raise the potential in the air gap at the points of intersection above the critical field stress value, which defines the value of electric field strength at which field discharge or breakdown occurs. It has been found that when there is such a discharge, a selective transfer or migration of charge occurs through the gap from the activated electrode or electrodes to the insulating medium. in the present invention, this discharge is selectively confined to only the points of intersection of the electrodes 33 and 35, for only at these points can the electric stress exceed the critical level. At all other points within the nip, the.

electric stress is below the critical level; therefore, dielectric breakdown is not effected and there is 'no charge transfer. lt is readily apparent, of course, that if the potential of the triggering pulse is sufficient, in itself, to exceed the critical field strength value, then prestressing of the insulating medium can be avoided.

The rotating contact electrodes of the present invention are subject to minimal wear. Advantageously, the counter-rotating rolls 29 and 3i are operated at the same rotational speed, thus there is no relative motion between the dielectric medium and the rolls nor is there any slippage of the dielectric medium. In this manner, although the rolls are in contact with the dielectric medium, the absence of relative motion therebetween avoids wear on the electrodes and frequent replacement thereof.

ln an alternate embodiment of the present invention shown in FIG. 2, the roll 29 containing electrode set 33 is replaced with a single electrode, preferably a stationary knife edge electrode 34, which is grounded as shown. Electrode 34 is spaced proximate roll 331 and is substantially coterminious therewith and positioned parallel with the axis of rotation of said roll. The electrode 34 and the electrode set 35 in roll 31 are adapted to make line contact with the dielectric medium 10 passing therebetween. Upon activation of electrode (0) of electrode set 35 in roll 31, for example, charge transfer occurs in the nip 27 at the cross-over point (y) of grounded electrode 34 and electrode (c). This embodiment of the present invention provides a low cost, rugged contact charging apparatus with a minimum of moving parts which nevertheless offers continuous operation, minimum maintenance and also minimizes wear on the multiple electrode roll.

What is claimed is:

1. An electrographic contact charging apparatus imparting to a dielectric medium surface an image, said apparatus comprising first and second rolls adapted to make line contact in the nip formed therebetween with said dielectric medium adapted to pass therethrough, said first roll having a plurality of spaced electrodes in the surface thereof, said electrodes being disposed concentrically with the axis of rotation; said second roll having a plurality of spaced electrodes in the surface thereof, said electrodes being disposed parallel with the axis of rotation, means for grounding the electrodes of said second roll upon entering the nip, means for generating sequential informational triggering pulses, and commutating means for connecting the electrodes of said first roll with said means for generating said triggering pulses, each of said triggering pulses causing transfer ofa single electrostatic point image to said dielectric medium at the point of virtual intersection of a spaced electrode receiving a triggering pulse and said grounded electrode.

2. An electrographic contact charging apparatus imparting to a dielectric medium surface an image, said apparatus comprising first and second rolls adapted to make-line contact in the nip formed therebetween with said dielectric medium adapted to pass therethrough, means including an ionization electrode for electrically prestressing said web to a potential below the critical stress value at which charge migration occurs, said first roll having a plurality of spaced electrodes in the surface thereof, said electrodes being disposed concentrically with the axis of rotation; said second roll having a plurality of spaced electrodes in the surface thereof,

said electrodes being disposed parallel with the axis ofv rotation, means for grounding the electrodes of said second roll upon entering the nip, means for generating sequential informational triggering pulses, and commutating means for connecting the electrodes of said first roll with said means for. generating said triggering pulses, each of said triggering pulses providing an electrostatic field sufficient to exceed said critical stress value causing transfer of a single electrostatic point image to said dielectric medium at the point of virtual intersection ofa spaced electrode receiving a triggering pulse and said grounded electrode.

3. Process for continuous electrographic recording comprising:

i. passing an insulating web through the nip formed by a first roll containing a plurality of spaced electrodes in the surface thereof, said electrodes being disposed concentrically with the axis of rotation of said roll and at least one grounded electrode spaced from said roll and parallel with the axis of rotation thereof and extending substantially coterminously with said roll, said grounded electrode comprising a second roll containing a plurality of pulses applied to the electrodes in said roll; and iii. developing said electrostatic latent images. 4. Process as defined in claim 3 wherein the insulating web is initially prestressed to a potential below the 5 critical stress value at which charge migration occurs upon passage through a zone containing an ionization electrode.

5. Process as defined in claim 3 wherein the developed image is subsequently fused in place. x =1= a: 

1. An electrographic contact charging apparatus imparting to a dielectric medium surface an image, said apparatus comprising first and second rolls adapted to make line contact in the nip formed therebetween with said dielectric medium adapted to pass therethrough, said first roll having a plurality of spaced electrodes in the surface thereof, said first roll electrodes being disposed concentrically with the axis of rotation; said second roll having a plurality of spaced electrodes in the surface thereof, said second roll electrodes being disposed parallel with the axis of rotation, means for grounding the electrodes of said second roll upon entering the nip, means for generating sequential informational triggering pulses, and commutating means for connecting the electrodes of said first roll with said means for generating said triggering pulses, each of said triggering pulses causing transfer of a single electrostatic point image to said dielectric medium at the point of virtual intersection of a spaced electrode receiving a triggering pulse and said grounded electrode.
 2. An electrographic contact charging apparatus imparting to a dielectric medium surface an image, said apparatus comprising first and second rolls adapted to make line contact in the nip formed therebetween with said dielectric medium adapted to pass therethrough, means including an ionization electrode for electrically prestressing said web to a potential below the critical stress value at which charge migration occurs, said first roll having a plurality of spaced electrodes in the surface thereof, said first roll electrodes being disposed concentrically with the axis of rotation; said second roll having a plurality of spaced electrodes in the surface thereof, said second roll electrodes being disposed parallel with the axis of rotation, means for grounding the electrodes of said second roll upon entering the nip, means for generating sequential informational triggering pulses, and commutating means for connecting the electrodes of said first roll with said means for generating said triggering pulses, each of said triggering pulses providing an electrostatic field sufficient to exceed said critical stress value causing transfer of a single electrostatic point image to said dielectric medium at the point of virtual intersection of a spaced electrode receiving a triggering pulse and said grounded electrode.
 3. Process for continuous electrographic recording comprising: i. passing an insulating web through the nip formed by a first roll containing a plurality of spaced electrodes in the surface thereof, said electrodes being disposed concentrically with the axis of rotation of said roll and at least one grounded electrode spaced from said roll and parallel with the axis of rotation thereof and extending substantially coterminously with said roll, said grounded electrode comprising a second roll containing a plurality of spaced electrodes in the surface thereof, said second roll electrodes being disposed parallel with the axis of rotation, and means for grounding the electrodes of said second roll upon entering the nip formed between said first and second rolls; ii. providing electrostatic latent images formed of an arrangement of like point images on the web by selective charge transfer of point images between the electrodes in said first roll and said grounded electrode generated by sequential informational triggering pulses applied to the electrodes in said first roll; and iii. developing said electrostatic latent images.
 4. Process as defined in claim 3 wherein the insulatIng web is initially prestressed to a potential below the critical stress value at which charge migration occurs upon passage through a zone containing an ionization electrode.
 5. Process as defined in claim 3 wherein the developed image is subsequently fused in place. 